1. Field of the Invention
The present invention relates to an asynchronous speech data communication system and a communication method therefor and, more particularly, relates to a communication system for making a hands-free phone conversation in a vehicle.
2. Description of the Related Art
When speech transmission is performed in a vehicle, a hands-free phone conversation is generally used from the viewpoint of convenience. For example, when a hands-free phone conversation is made using a mobile phone, a user uses a vehicle-mounted audio device and an input/output terminal incorporated in a navigation device or uses a hands-free terminal in which a headphone and a microphone are installed in order to communicate speech data in a wireless manner between the terminal and the main unit of the mobile phone.
In wireless communication, BlueTooth is used as a short distance wireless data communication technology. In BlueTooth, transmission and reception of data such as speech is performed in a wireless manner among mobile phones, notebook computers, PDAs (Personal Digital Assistants), etc. The frequency band used is a 2.45-GHz radio frequency (RF), the operating range is within approximately 10 m, and the data transfer rate is approximately 1 Mbps.
Since the data communication speed of BlueTooth is not very high, as an alternative technology, communication using a wireless LAN (Local Area Network) has begun to be used.
For example, Japanese Unexamined Patent Application Publication No. 2001-136190 discloses a technique in which, in order that an AV device in a vehicle can be used in another vehicle, a wireless LAN unit is connected to a LAN system in the vehicle, and the LAN systems of the vehicles are linked via the wireless LAN unit.
However, the conventional in-vehicle hands-free phone conversation using wireless technology has the following problems. Since BlueTooth has both an asynchronous data channel for data communication and a synchronous channel for speech communication, BlueTooth can be used without problems even for an application that is sensitive to speech delay, such as a hands-free phone conversation. However, when this is to be replaced with a wireless LAN, since the wireless LAN has only an asynchronous data communication system, some kind of mechanism for permitting the maximum delay time in an application that is sensitive to speech delay, such as a hands-free phone conversation, becomes necessary.
When speech data is transmitted by a wireless LAN, the speech data is subjected to pulse code modulation (PCM), the coded speech data is packetized, and this packetized speech data is transmitted to an access point. For the access control system for a wireless LAN, CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is used. In this method, when another terminal is communicating with the wireless LAN, the transmission of speech data must be postponed until the communication of the other terminal is completed. That is, the larger the size of one data frame (packet), the larger the maximum waiting time.
FIG. 10 shows the format of a physical layer for use in a wireless LAN (direct diffusion method). FIG. 11 shows the relationship between each bit ratio when the MPDU (data) is the maximum number of bits (65536 bits) and the transmission time at that time.
The frame format has a preamble for achieving synchronization among devices, a header for addresses of a destination and a transmission source and lengths thereof, and a data unit (MPDU) containing data of a variable size. The variable range of data is 4 to 8192 bytes (32 to 65536 bits). When the bit rate of the wireless LAN is set to 1 Mbps to 11 Mbps (54 Mbps has also been used in practice), the delay time, that is, the waiting time, becomes a maximum of approximately 65 ms at the lowest bit rate of 1 Mbps.
Furthermore, since a situation is assumed in which there are two or more terminals waiting for a transmission (the waiting time in this case is 65 ms □ the number of frames), it is not ensured that the speech data can be transmitted in the waiting time of the frame from one terminal. For example, as shown in FIG. 12, when there are two or more terminals that perform data communication with the access point of the in-vehicle wireless LAN, for example, when a PDA, a mobile audio, and a mobile phone exist, speech data V cannot be transmitted from the mobile phone to the access point while the PDA or the mobile audio is transmitting data to the access point. As described above, the waiting time Tmax for transmitting the speech data V is proportional to the data size from the PDA and the mobile audio, that is, the packet size, and during that time, the transmission of the speech data must be postponed. When the delay of the speech data reaches a fixed level or higher, the speech transmission quality deteriorates, and the other party with whom communication is performed may experience some annoyance.